Tubular metallic structures are widely used in various industries including manufacturing and constructions for carrying loads or providing supports. Efforts have been made to improve the strength of these tubular metallic structures to enhance safety and stability. The improvement in the strength of the tubular metal structures also assists in replacing bulky and heavy metallic tubes with smaller and lighter tubes, and thus reducing the overall size and weight of the resulting products or structures.
Surface treating or processing is a convenient method for improving strength a structure, and particularly, a metallic structure. In 1999, the process of Surface Mechanical Attrition Treatment (SMAT) is first proposed by K. Lu and J. Lu, and since then the process has attracted increasing interests in the field. SMAT is an efficient method to create a layer of nano-crystallized structure on the surface of metals. Balls having a smooth, spherical surface generally made of stainless steel, tungsten-carbide and ceramics, etc., are placed in a working chamber along with a metallic sample to be surface-treated. The balls are then made to vibrate to resonance by a vibration generator, and that the sample is then subjected to collision by a large number of fast moving balls over a short period of time. Each collision creates an impact which induces plastic deformation with a high strain rate to the metal surface of the sample. As a consequence, the repeated multi-directional impacts at high strain rate onto the sample surface result in numerous plastic deformations and grain refinements, which progressively down to the nanometer regime over the entire sample surface and provides a significant enhancement on the strength of the surface being treated.
SMAT has been proved successful in enhancing strength of a planar surface or an outer surface of a tubular structure. However, treatment on an inner surface of a tubular structure remains a critical problem which significantly affects the efficacy of the treatment and thus the strength of the resulting structure. Accordingly, there has been a continual need for an effective and simple method in processing an inner surface of a tubular structure.